Studies conducted over the last twenty-five years have suggested that a substantial fraction of hospital use is inappropriate. As much as a quarter of acute hospital care is reported to be unnecessary, when medical records are reviewed. Further suggestive evidence of inappropriate hospitalization appears in studies that have found large geographic variation in surgery rates and in hospital admission rates by diagnostic category. These variations are often used to suggest that unnecessary or inappropriate medical services are being provided in high use areas, and that health care costs could be reduced substantially if medically unnecessary inpatient use were curtailed. This grant will examine the relationship between provider (e.g., physician and hospital) characteristics and inappropriate hospital use, in terms of both admissions and days. The study will use longitudinal data from a randomized trial of health insurance, the Rand Health Insurance Experiment. That study provides a unique opportunity to assess inappropriate inpatient use. The data include evaluations of the appropriateness of inpatient treatment based on medical record review, patient characteristics (sociodemographic, economic, and health status), and provider characteristics (specialty and practice pattern for physicians, accreditations and facilities for hospitals). This grant builds on past work with these data by this research team, focusing on the effects of geographic variation and cost- sharing on inappropriate inpatient use.